Structure of Normal Peripheral Nerves. Kranial Nerves, Spinal Cord and Rhombencephalon
Peripheral nerves and cranial nerves as well as the spinal cord constitute a functional unit which is responsible for the movements of the body. The unit includes also sensory impulses to the cerebrum and the cerebellum. When undamaged, the unit provides information directed inwards and outwards which is a prerequisite for our daily life.
Each neuron consists of a cell body (perikaryon or soma) as well as two types of processes, one which receives information (dendrites) and one which delivers information (axon) as electrical impulses. The axons of peripheral nerves and cranial nerves are contained within Schwann cells which produce myelin, an insulating lipid-rich material which isolates each axon from the others. In the spinal cord and the rhombencephalon, oligodendroglial cells surround the axons and form the myelin sheaths. In addition, other cell types, such as astrocytes and microglial cells, are present in the spinal cord.
Astrocytes make up 50-70% of the volume occupied by glial cells in the brain. They maintain the microenvironment of the neurons. The microglial cells are much fewer. They are activated during damage to the brain and phagocytize dead material. Furthermore, they play a key role in the immune system of the brain.
Organization of the Peripheral Nerves and the Spinal Cord
The peripheral nerves are made up of axons which mediate impulses to the muscles which in turn cause contractions (motor axons) and axons which mediate information from muscle spindles, tendons joint capsules and the skin (sensory axons). The motor axons are the processes from large perikarya localized in the anterior horns of the spinal cord. The sensory axons have their perikarya in the dorsal root ganglia where they are surrounded by satellite glial cells. The sensory neurons in the spinal cord transfer the information further to the cerebellum and cerebrum.
Structure of the Spinal Cord
The neuronal perikarya of the spinal cord are organized as grey matter which, in the cross section, is shaped like an H. This is surrounded by white matter which contains axons travelling up or down.
Structure of Rhombencephalon and the Cranial Nerves
Ten out of the twelve cranial nerves have their perikarya located in groups (nuclei) within the rhombencephalon. The cranial nerves have either a motor or a sensory function.